Firefighter Alex Jimenez wades through the mud in Montecito after finding remains of a victim of the flash flood.
Photograph: Wally Skalij/Getty Images

Jeanette Abney owns a big, fancy house and Elizabeth Terry rents a room in a boarding house. But this week they both ended up sleeping on cots in the same American Red Cross evacuation centre, sipping the same instant coffee, nibbling the same pastries and huddling under the same blankets. A rain-sodden poster at the entrance declared “disaster services”.

Both women were in need. A storm had drenched the Verdugo mountains, a rugged, rustic outpost of Los Angeles, and unleashed a massive mudslide, forcing them to flee to an improvised evacuation centre in the San Fernando valley.

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“It was like a war zone, like Niagara Falls. I’ve never experienced anything like that,” said Abney, 88.

She counted herself lucky to be safe and warm in a shelter. So did Terry, 63, who lives a few miles from Abney in a boarding house for women on the verge of homelessness. “The mud was pouring down the hill. I knew it was time to go.”

Rich and poor thrown together, if only briefly, by a force oblivious to class distinction: nature.

The rains and avalanches of mud and debris which struck chunks of southern California this week did not respect zip codes or security walls or camera-controlled gates.

The surges of rock and boulder smashed through the estates of billionaires and gritty, hardscrabble homes, sweeping away lives and property in brutal reminder that the golden state can exact a terrible price from those who choose to live here.

A land with four seasons, goes the old, bitter joke: drought, earthquake, fire and flood.

A tour of the Verdugo mountains this week revealed knee-high swamps of mud encasing homes and cars. Many slopes were black and blanketed with ash, the legacy of a wildfire last September.

Worst hit was Montecito, an affluent, bucolic town with celebrity residents to the north. It abuts vineyards in Santa Barbara county which last month endured the biggest wildfire in California history, an inferno which charred vegetation and left soil unable to efficiently absorb moisture. Torrential rains triggered calamity.

Residents awoke early on Tuesday morning to a roaring sound. Survivors would compare it to a freight train. Sludge thundered down hillsides in a roiling mash of boulders, trees and cars, cleaving houses from foundations, erasing entire blocks and burying the 101 highway.

Eighteen people died, including four children. The coroner has listed the cause of death for each as “multiple traumatic injuries due to flash flood with mudslides due to recent wildfire”. Rescue teams with dogs, helicopters and specialised vehicles plucked survivors from the ruins. “I thought I was dead for a minute there,” a 14-year-old girl told them.

The super-wealthy who live high on certain slopes emerged relatively unscathed. The less wealthy lower down the slopes bore the brunt.

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“I’m looking out the front window, I think everything’s fine,” Oprah Winfrey told Ellen DeGeneres in a call to her show. “It wasn’t until I put my boots on and went outside walking and I realized everything wasn’t indeed fine. All of my neighbours’ homes are like gutted. Their houses are gone, just gone.”

Winfrey, whose barnstorming speech at the Golden Globes last Sunday spurred chatter of a White House run, vowed to help Montecito rebuild. “We’re going to come together and do what great Americans do all the time. We’re going to help each other.”

That may sound a pious bromide but natural disasters often do soften America’s class and racial divisions, however fleetingly.

When Hurricane Harvey flooded Houston last August volunteers with canoes, skiffs and airboats from Louisiana – Trump country – rushed to aid one of America’s most diverse cities. “Put a gay or black person in need next to them and they’ll help. But in a voting booth they’ll turn around and cut their healthcare. It’ll just blow up your mind,” one boatman told me then.

There was a similar twist last year after landslides cut off Big Sur, a picturesque coastal ribbon north of Montecito. Wealthy visitors helicoptered in to a luxury resort to savour the temporary, enforced stillness of an area usually teeming with tourists.

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“We had a very negative opinion initially of the helicopter thing - they being the rich and we being the poor,” said John Hoeffel, 71, a resident. He changed his mind because the high-rolling visitors saved jobs, a view echoed by local Latino cooks, cleaners and chambermaids. “We all need jobs,” said Josue Ramos, 19, a gardener.

In Los Angeles County alone, the number of homeless spiked last year by 23% to nearly 60,000 people. Once confined largely to Skid Row, a grim tent city in the heart of downtown, the homeless have fanned out across the city, including Bel-Air, a neighbourhood of mansions etched into canyons.

Residents include Elon Musk and Rupert Murdoch, who has a winery. A developer is putting the finishing touches to a 100,000 square foot house which is due to soon go on sale for $500m, billing itself as America’s most expensive home.

Some sensed a parable at work last month when a cooking fire at a homeless encampment leaped out of control and scorched 500 acres of Bel-Air, destroying several properties and damaging dozens of others, including Murdoch’s.

Many of the city’s rich and poor are also united in a disregard for nature.

LA may be synonymous with urban sprawl but boasts canyons and mountains which offer rural seclusion and spectacular views.

Those who can afford it build – with the complicity of developers and local authorities – homes in ever-more rugged areas. Those who can’t afford to build camp or park mobile homes there.

A search and rescue dog is guided through the mud in Montecito. Photograph: Kyle Grillot/Reuters

“Dude, why wouldn’t I live here? It’s gorgeous,” said one homeless resident in a remote corner of of Bel-Air, who gave his name only as Carlos. “Skid Row’s a germ factory.”

As people move up the slopes a pattern repeats: drought parches the land, turning grass into kindling. Hot autumn winds known as Santa Anas fan wildfires, which turn soil to dust. Rain turns slopes into mush and unleashes mudslides.

Winfrey, in her call to DeGeneres, echoed surprise that one calamity should so swiftly follow another. “After we survived the fires and the rain came who would have expected that we would have this devastation again, with the mudslides so soon?”

Well, authorities expected it and issued evacuation orders in Montecito which were largely ignored, with many citing “disaster fatigue”.

“Why do people live here?” John McPhee asked in a landmark 1988 New Yorker essay, Los Angeles Against the Mountains, which formed the basis of a book, The Control of Nature. His answer: “They would rather defy nature than live without it.”

He quoted a homeowner who was asked in a 1964 newspaper article why he lived in the Verdugo mountains. “There isn’t a prettier, more secluded canyon in southern California – when it isn’t on fire or being washed away. Each time we have a disaster, only one or two families move out, but there are hundreds standing in line waiting to move in. People live here, come hell or high water. Both come, and we still stay.”

Back at the Red Cross evacuation centre I asked Abney if her narrow escape would persuade her to move home – to leave the mountain. She shook her head. “Oh no.”